The present invention relates generally to the field of radiation contamination detection systems and, in its most preferred embodiments, to the field of position sensitive proportional counters.
In order to achieve a lower limit of detection for radioactive contamination detection systems, conventional contamination detection systems (laundry monitors, portal monitors, floor monitors, etc.) typically employ multiple separate radiation detectors in separate housings or separate radiation detectors located in a common housing. In these systems, each separate detector has a set of electronics dedicated solely to it. To align these systems, a technician must assure proper initial conditions (power supply voltages, etc.) and set amplifier gain, discriminator level (or levels for systems that measure more than one type of radiation) and high voltage for each separate detector to place the system into proper operating condition. In order to calibrate one of the detectors in the system, a measurement of the efficiency must be made, using a standardized source of radiation. This work must be repeated for each of the separate detectors. The number of separate detectors in a system can exceed 50, therefore the initialization and calibration of these systems can be quite cumbersome. Thus, it is not an easy matter to calibration these conventional systems. Calibration is typically performed at least annually, or when the system is found to be inoperable. Due to the infrequency of calibrations, high quality electronics must typically be used in conventional systems to minimize the effect of drifts from temperature, supply voltage changes and aging effects between calibrations.
Conventional contamination detection systems, having separate detector housings with discrete separate detectors therein, typically incorporate computers which test for gross system failure. The computers cannot identify system drifts, even if the system is no longer capable of detecting radiation at some predefined regulatory limit. To compensate for this, most sites with contamination monitoring systems perform a daily check that places a source of radiation near each of the separate detectors to confirm that the monitor operates properly and alarms. Regulatory agencies often require these checks to be quantitative. If the monitor fails to alarm, the system must be taken out of service for repair and re-calibration. Many man-hours are required to keep a single system operating due to the necessity of "checking" the discrete set of electronics associated with each separate detector. Additionally, if a system fails a daily source check, there is an ambiguity about when the failure occurred, and how much material was released without proper monitoring for contamination.
In one sense, monitoring is ideally accomplished by a large number of adjacent, small, separate detectors. This reduces the background count rate per detector and thus reduces each detector's, and thus the overall system's, detection limit. However, because of the man-hours required to calibrate, maintain and source check each separate detector in conventional contamination monitoring systems, and because of the cost of the electronics for each detector, it is prohibitive to sufficiently increase the number of separate detectors. Additionally, in conventional contamination monitoring systems sensitivity is decreased in the regions between detectors. Thus, there are limitations to the number of detectors that can feasibly be employed, and thus limitations to conventional contamination monitoring systems.